(1.) Field of the Invention
This invention relates to a power generating type velocity sensor for detecting a velocity, for example, the velocity of a movable lens of a camera.
(2.) Description of the Prior Art
A number of mechanisms are known for focusing or zooming operations of a camera lens system. One example of such mechanisms is illustrated in FIG. 5. As shown in the figure, a stationary lens 2 is disposed at the center of a housing 1, on which is mounted an actuator 4 having an annular magnet 3 disposed concentrically with the stationary lens 2. On the other hand, a group of movable lenses 6 is mounted on a cylindrical portion 5a central to a movable lens holding member 5, and is inserted in the actuator 4. The cylindrical portion 5a is provided thereon with a coil 7 opposed to the inner peripheral surface of the magnet 3 so that the movable lens holding member 5 is moved in the axial direction when the coil 7 is energized. The group of movable lenses 6, arranged on the same optical axis as that of the stationary lens 2, is moved to thereby achieve the desired focusing or zooming operation of the lens system.
A stick-shaped magnet 8, magnetized to have opposite magnetic poles (N and S poles) at longitudinal ends thereof, is mounted on the outer periphery of the movable lens holding member 5, parallel to the axial direction of the movable lens holding member 5. On the other hand, a cylindrical coil 9 is mounted on the housing 1 at a position facing to the magnet 8 so that the magnet 8 is moved inside the coil 9 when the movable lens holding member 5 is moved in the axial direction. The magnet 8 and the coil 9 constitute a velocity sensor 10 which detects the velocity of the movable lenses 6 based on a counter-electromotive force arising from the attendant relative movement of the magnet 8 and the coil 9.
In the velocity sensor 10 of the above type, the coil 9 has conventionally been wound in a uniform configuration, as shown in FIG. 6 , so that the number of turns per unit axial length of the coil 9 is uniform throughout the axial length of the coil 9. A movement of the magnet 8, with the coil 9 kept stationary, produces a counter-electromotive force e proportional to the rate of change in the magnetic flux interlinkage (or flux linkage) .phi. through the coil 9. Hence e=d.phi./dt. The magnetic flux generated by the magnet 8 broadens, in cross section, from the longitudinal ends toward the longitudinal center of the magnet 8, as illustrated in FIG. 7 . In the conventional velocity sensor 10 with a uniformly wound coil, therefore, the variation in the magnetic flux interlinkage is smaller at the ends than at the center of the coil 9. Consequently, where servo constants are so determined as to obtain an optimum output of counter-electromotive force e at the center of the coil 9, the counter-electromotive force induced during a constant-velocity movement is decreased at the ends of the coil 9, as indicated by curve a in FIG. 2 , and response speed is lowered accordingly.